Structural nonuniformities in vehicle tires can give rise to an undesirable phenomenon known as "steering pull". Steering pull refers to the tendency of a vehicle traveling forward to veer to the right or left in the absence of a corresponding steering input from its driver. Steering pull can be quantified by measuring the magnitude and direction of force which the driver must apply at the perimeter of the steering wheel in one direction or the other in order to keep the vehicle traveling straight ahead.
The steering pull exhibited by a vehicle due to its tires is attributable almost entirely to behavior of the tires mounted on the steering axle. The tendency of a tire on one end of the axle to cause steering pull algebraically adds to or subtracts from the corresponding tendency of the tire on the other end of the axle. In order to illustrate this, it is useful to first note that tires have a pair of opposed sides which are commonly referred to as the "in-side" (or D.O.T. marking side) and the "out-side" (sometimes referred to as the "curb side" or "whitewall side"). Assume for illustration purposes that the tire mounted on the left side of the front axle of a vehicle, such as a passenger car, tends to veer toward its out-side to such a degree that in order to keep the vehicle traveling straight, a 5 ounce clockwise steering pull force must be applied by the driver to the perimeter of the steering wheel of the car. Further assume that the tire on the opposite end of the same axle tends to pull in the opposite direction, i.e., toward the in-side, with a steering pull of the same magnitude. Since the tires are mounted at opposite ends of the axle, each would tend to cause the car to tend to veer leftwardly and, in order to keep the vehicle traveling straight, the driver would have to exert a distinctly noticeable total steering pull of 10 ounces on the steering wheel in the clockwise direction. However, if only one of these tires was replaced with a tire exhibiting the same degree of steering pull but in the opposite direction of its predecessor, i.e., in the same direction as the tire on the opposite end of the steering axle, the driver would not have to exert any force on the steering wheel to keep the vehicle traveling straight ahead. No annoying steering pull would then be apparent to the driver notwithstanding the tendency of each individual tire to cause steering pull.
One technique for reducing steering pull known in the prior art is to use a tire uniformity machine to measure in tires a parameter, such as conicity, known to be correlated to the tendency of an individual tire to contribute to steering pull and to reject any tire whose conicity exceeds a predetermined maximum limit. However, this technique, when practiced alone, is only effective to limit the maximum contribution to steering pull which can be made by a given tire. Moreover, attempts to reduce such maximum contribution by lowering the reject limit on conicity are gained only at the expense of increased numbers of rejected tires thereby decreasing productivity and raising production costs.
As its name implies, "conicity" indicates the tendency of a tire to roll in an arc as would a section of a cone rather than straight ahead as a true cylinder would tend to roll. Conicity can be measured substantially simultaneously with other non-uniformity indicating parameters on a conventional tire uniformity machine and is commonly expressed as an algebraically signed (+ or-) number in units of pounds or Newtons. Conventionally, positive conicity values indicate a tendency to pull toward the out-side of the tire while negative conicity values indicate a tendency to pull to the in-side.
While it is conventional practice to use conicity as an index for steering pull, conicity is not the only parameter known to be correlated to the tendency of a tire to cause steering pull. Some research suggests that steering pull is caused primarily by the aligning moment of the tire (the moment around the radial axis). Papers such as SAE 870423, "Tread Design and Belt Angle Effect on Residual Aligning Torque", by Frank E. Matyja, have shown that there is a correlation between conicity and residual aligning torque which is the aligning torque existing when the tire rolls so as to generate zero lateral force.
Another known technique for reducing steering pull, which can be practiced alone or in combination with the limiting technique just described, is to mark tires with the algebraic sign of their conicities and select only tires of the same algebraic sign for mounting on opposite ends of the steering axle of a given vehicle. As a consequence, the tendency of each tire to cause steering pull detectable at the steering wheel of the vehicle tends to be at least partially cancelled or offset by the opposing action of the tire on the opposite end of the axle. While beneficial, the degree of cancellation achieved using such a system is uncontrolled and substantial variations in steering pull from one set of tires so selected to another can occur.
In view of the foregoing, it is an objective of the invention to provide a process and apparatus for selecting a set of at least two tires for mounting on the same vehicle such that the tires in a given set tend to contribute to steering pull not only in the same direction with respect to each tire, but also with comparable magnitudes so as to achieve significant cancellation of the steering pull sensed by the driver at the steering wheel with greater consistency than has been possible heretofore.
Another objective of the invention is to provide a selection process which, in addition to meeting the foregoing objectives, can be carried out as a continuous process wherein tires or tire/wheel assemblies are continuously taken up to be measured and properly selected sets of same are continuously delivered.
Another objective is to provide such a continuous process and apparatus which can be economically implemented on a production scale and which do not require facilities for physically storing excessive numbers of tires or tire/wheel assemblies in the interval between measuring their individual tendencies to contribute to steering pull and selecting sets thereof.
It is a further objective of the invention to provide a set of tires or tire/wheel assemblies so selected so as to fulfill the foregoing objectives as well as an additional objective.
That additional objective of the present invention is to provide a selection process and apparatus which can be combined with the invention of the parent Ser. No. 07/556,951 application so as to concurrently achieve the advantages thereof in a single, integrated process and apparatus.
Parent U.S. patent application Ser. No. 07/556,951 is directed to a process and apparatus for pairing tires from a given population of tires with wheels from a given population of wheels so as to optimally reduce radial force variation in the resulting population of tire/wheel assemblies. According to that invention, tires and wheels are inspected as individual components by measuring in each a parameter correlated to its tendency to generate vibration in a given direction. Each tire and wheel in respective groups of N tires and N wheels so inspected are then assigned rank designations such as an ordinal number in the range of 1 through N. The rank designations are assigned in order of the values of the parameters measured and thus indicate relative tendency of each given tire and wheel to generate such vibration as compared to the remaining N-1 members of its respective group. Tires and wheels of corresponding rank designation are paired for mounting as a tire/wheel assembly. In the tire/wheel assembly, the tire and wheel are oriented with respect to one another so as to minimize the aforementioned vibration. That system however does not address the problem of steering pull.